Create a docker container to be used headless

[wil3]

The current docker file, tested for OSX, supports the Gazebo front end. However to do this requires the Mesa library which takes a considerably long time to build. For training a controller there is no need to include these libraries.

Create a base image for headless use and then derive a second image with a front end.

[SwapnilPande]

As I mentioned in the other issue, I have already started building a headless container. If we can identify the root cause in that issue, I can build a Dockerfile for that configuration.

[wil3]

With the dependency script looking good I'm going to update the current docker image used for Travis to use the same script so the environments are all the same.

[wil3]

Related to Issue: #58